For applications in electrical engineering, such as measurement transformers or supply transformers, magnetic cores are used which contain of a magnetic material chosen for its magnetic properties, such as its magnetic permeability or its losses. For these applications, the shape of the hysteresis loop is not essential. On the other hand, for many applications handling low-amplitude electrical signals, for example residual current devices, switching-mode power supplies or transformers for connecting to digital telephone networks, the shape of the hysteresis loop is of paramount importance. The shape of the hysteresis loop is characterized, in particular, by the B.sub.r /B.sub.m ratio--the ratio of the remanent induction to the maximum induction. When B.sub.r /B.sub.m is greater than approximately 0.9, the hysteresis loop is called "rectangular". When the B.sub.r /B.sub.m ratio is less than approximately 0.5, the hysteresis loop is called "flat". Materials having a rectangular hysteresis loop are used, for example, to produce the magnetic cores of magnetic amplifiers or of control stages for switching-mode power supplies. Materials having a flat hysteresis loop are used, in particular, for producing the magnetic cores of residual current devices, electrical filters or DC isolating transformers.
In order to manufacture magnetic components from a soft magnetic material having a precisely shaped hysteresis loop, either rectangular or flat, soft magnetic alloys having low anisotropy (anisotropy coefficients of less than 5000 ergs/cm.sup.3 and preferably less than 1000 ergs/cm.sup.3) are used, such as 15/80/5 FeNiMo alloys, amorphous Co-based alloys or nanocrystalline FeSiCuNbB-type alloys, and the magnetic components are annealed in an intense magnetic field. The annealing is carried out at a temperature below the Curie point of the alloy. The magnetic field is longitudinal, i.e. parallel to the direction in which the magnetic properties will be measured, when it is desired to obtain a rectangular hysteresis loop. It is transverse, i.e. perpendicular to the direction in which the magnetic properties will be measured, when it is desired to obtain a flat hysteresis loop. The magnetic field is applied throughout the duration of the treatment, and it is constant. The temperature and duration of treatment are the two parameters which have an impact on the result of the heat treatment. These treatments, when they are of long duration (from one hour to a few hours), make it possible to obtain, with great reliability, either very rectangular (B.sub.r /B.sub.m &gt;0.9) hysteresis loops or very flat (B.sub.r /B.sub.m &lt;0.2) hysteresis loops. However, they do not make it possible to obtain with sufficient reliability hysteresis loops having an intermediate shape (0.3&lt;B.sub.r /B.sub.m &lt;0.9) which are very useful for some applications. This is because, in order to obtain such hysteresis loops, it is necessary to carry out short annealing operations, but then the results are much too random, both in terms of rectangularity and permeability, to be able to envisage an industrial application. This is because both these parameters must be able to be controlled simultaneously.